1. Technical Field
The present invention pertains to improvements for envelopes containing a fill for use in electrodeless lamps and has particular, although not limited, utility in lamps of the type disclosed in U.S. Pat. No. 5,404,076 and PCT International Publication No. WO 92/08240, the disclosures of which are expressly incorporated by reference herein in their entirety.
More particularly, the present invention is directed to an improved electrodeless sulfur or selenium lamp which does not require bulb rotation. The present invention further relates to electrodeless discharge lamps for exciting fills in electrodeless lamp bulbs with circular polarized microwave energy.
2. Related Art
Recently, a new lamp providing visible radiation, known as the sulfur lamp, or selenium lamp, depending on which substance is used, has been introduced. This lamp provides a superior spectral output at high efficacy, and enjoys a very long lifetime in, for example, an electrodeless lamp environment.
In these lamps, sulfur, selenium, or both as the case may be, is provided in a lamp bulb in an amount sufficient, when suitably excited, to provide principally molecular radiation in the visible region of the spectrum. The lamp, which typically includes a bulb which is rotated during operation, is described in detail in the above-referenced publications and also in PCT publications WO 95/10848, WO 96/28840, WO 96/33509, and WO 97/27609, and U.S. Pat. Nos. 5,594,303, and 5,688,064, each of which is incorporated herein by reference in its entirety.
PCT Publication No. WO 94/08439 (the ""439 Publication) discusses the desirability of rotation at sufficient speeds in order to fill the interior of the bulb with an arc discharge, i.e., to prevent isolated discharges from occurring.
For many applications, it would be desirable to operate the lamp without bulb rotation. For example, the motor required for rotation adds expense to the system and reduces reliability. Since the electrodeless bulb has a very long lifetime, the motor is apt to fail before the bulb, thus requiring maintenance and/or replacement which would otherwise be unnecessary.
In accordance with the present invention it has been discovered that in an excited fill which would otherwise require rotation to stabilize the arc, an alkali metal present in the excited fill stabilizes the arc without bulb rotation. The alkali metal may be introduced in the unexcited fill in the form of a halide, and cesium is the most practical of the alkali metals. Cesium bromide is a compound which may be utilized.
The invention also provides an unexpected advantage in that the cesium has the effect of altering the spectral output of the lamp in a positive way. Thus, it has been found that the color rendering index (CRI) of a lamp including cesium is higher than in its absence, providing a desirable higher red to blue ratio of emitted light.
In the absence of bulb rotation, the fill is preferably excited by a non-stationary electric field in order to spread the discharge out, minimize hot spots, and prolong bulb life. A non-stationary electric field is an electric field having a direction which changes, with respect to a fixed location on the bulb, during lamp operation. For example, the fill is preferably excited by circular polarized microwave energy.
In accordance with one aspect of the invention, circular polarization is provided from a microwave circuit inserted between a source of microwave power and a cylindrical cavity containing an electrodeless lamp. For example, an electrodeless microwave discharge lamp is provided with a waveguide coupling structure for coupling an electromagnetic wave from a single aperture in a rectangular waveguide to a cylindrical waveguide containing an electrodeless lamp bulb. The waveguide coupling structure includes one end having an aperture connected to the single aperture of the rectangular waveguide, and another end which is connected to a cylindrical waveguide. The waveguide coupling structure creates two modes of electromagnetic radiation at the end which connects to the waveguide from the microwave radiation received from the rectangular waveguide. The two modes of electromagnetic radiation have a phase velocity which differs, and at the point of coupling to the cylindrical waveguide are out of phase by 90xc2x0. The microwave radiation incident to the waveguide is circularly polarized by virtue of the phase difference between the two modes of electromagnetic radiation, and provides a rotating electric field around a longitudinal axis of the cylindrical waveguide. When an electrodeless lamp is supported along the axis of the cylindrical waveguide, the lamp plasma is more evenly excited, creating a more uniform temperature distribution about the circumference of the lamp envelope.
The waveguide coupling structure may be configured from a rectangular waveguide section, which has first and second sectional dimensions to provide a different phase velocity to first and second orthogonal modes of electromagnetic radiation. The height of the rectangular waveguide is selected so that a substantially 90xc2x0 phase difference exists between the two modes at the point where it is coupled to the cylindrical waveguide. In yet other embodiments of the invention, a dielectric material may be supported in a plane of a waveguide section, perpendicular to the plane of the rectangular waveguide single aperture which supplies the electromagnetic wave. The dielectric material induces a different phase velocity for first and second modes of coupled microwave electromagnetic radiation.
Other embodiments of the invention employ a microstrip antenna structure which is placed in a cylindrical waveguide structure, connecting the rectangular waveguide single aperture to the cylindrical waveguide having the electrodeless lamp. The microstrip antenna generates a circular polarized electric field which excites an electrodeless lamp.